One-part, moisture-curable silicone compositions are well known in the art. These systems generally comprise a silicone polymer having hydrolyzable silyl functionality and a catalyst, inter alia. Upon exposure to a moist atmosphere, the silyl groups react with water to form silanol groups which, in turn, condense to form a cured siloxane network, the condensation being facilitated by the catalyst. Since the silicone polymers characteristically exhibit relatively high moisture permeability, these systems typically cure to a depth of more than 10 mm after a week-long exposure to a moist atmosphere. Such rapid deep-section cure is often desired in certain sealant or adhesive applications (e.g., construction, automotive, do-it-yourself market, insulating glass).
On the other hand, a similar cured product based on a hydrocarbon polymer composition offers the advantage of having a low permeability, making it an attractive candidate for gas barrier applications such as form-in-place gaskets, O-rings, rubber plugs/seals, medical and food container caps, and the like. Unfortunately, this advantage is largely overshadowed by the sluggish cure exhibited by these systems. Generally, these systems only develop a surface cure (e.g., less than I mm) within a week owing to the slow transport of water to the composition's interior relative to the above described silicones.
The undesirably slow cure rate of moisture-curable hydrocarbon-based compositions can, of course, be overcome by providing an internal moisture source in the form of a two-part composition. For example, Japanese Publication No. 2-185565 to Kanegafuchi Kagaku Kogyo Kabushiki Kaisha discloses a curable composition to provide cured product having good weather and heat resistance. This composition contains a saturated hydrocarbon polymer having at least one reactive silicon-containing group and a hydrate of a metal salt, the latter providing the water source needed to obtain a good deep-section cure. However, such a two-part system has severe disadvantages in commercial applications since it must be mixed just prior to use, requires additional packaging and often results in wasted product. Similarly, EP 0839864, also to Kanegafuchi, discloses a composition comprising such a silyl-functional polymer, a saturated hydrocarbon oligomer and an ester plasticizer. This invention does not contemplate a storage-stable, one-part moisture curable system and the examples only illustrate two-part compositions wherein water or a metal salt hydrate (i.e., the moisture source) and curing catalyst were compounded as separate components and mixed prior to cure.
In another approach, the cure rate of a moisture-curable hydrocarbon polymer composition can be improved by the inclusion of a component which facilitates the diffusion of water to the interior. Thus, Japanese Publication No. 1-252670 to Kanegafuchi Kagaku Kogyo Kabushiki Kaisha discloses a hydrolyzable silyl-functional polyisobutylene (PIB) of the type taught by Iwahara et al. in U.S. Pat. No. 4,904,732 in combination with an organosilicon polymer, the latter component apparently being responsible for augmenting transport of water and resulting in improved cure rates. However, such a silicone component is notorious for imparting poor paintability to the resulting cured surface (i.e., silicones generally decrease surface energy and make painting with conventional paints extremely difficult). Moreover, the silicones explicitly illustrated in the 1-252670 publication are silanol-functional polymers, such as hydroxyl-terminated polydimethylsiloxane. A composition which includes such a component could not be stored as one-part system since the silanol groups of the organosilicon polymer would react with the hydrolyzable groups on the PIB and therefore tend to gel the resulting network or at least react into the network.
Alternatively, U.S. Pat. No. 4,808,664 to Saam discloses a moisture-curable polyisobutylene having hydrolyzable silylsiloxy terminal groups. The inventor teaches that the compositions are preferably cured to form thin films, but also suggests that thicker films can be prepared by including up to about 20 weight percent of a "moisture-transporting agent" in the curable composition. The only such agent mentioned is ethyl orthosilicate and no example illustrating its utility is presented. Moreover, a compound like ethyl orthosilicate can also react with moisture and react into the network to increase the crosslink density thereof and such a large quantity would reduce the moisture transport rate of the surface skin which forms as the system cures. Further, such compounds are not readily miscible with hydrocarbon polymers and would tend to phase separate upon storage.
In addition to the above publications, various other ingredients have been generally disclosed as optional components in moisture-curable systems. For example, U.S. Pat. No. 4,435,536 to Kato et al. discloses one-pack, moisture-curable compositions based on hydrolyzable silyl group containing vinyl polymers wherein the inclusion of a solvent imparts storage stability and "workability" to the compositions. When cured, the compositions are said to have excellent surface hardness and weather resistance. The only specific limitation on the solvent is that it does not lead to precipitation when combined with the vinyl polymer and a curing catalyst and it can, therefore, be a hydrocarbon, alcohol, ketone, ether or ester, inter alia. Preferred solvents include alcohols and hydrolyzable esters, methanol, methyl orthoformate, ethyl orthoformate, methyltrimethoxilane and ethyl silicate being specifically illustrated. However, the systems illustrated appear to be cured at elevated temperatures as thin films and are not shown to provide deep-section cure. Additionally, the solvents illustrated either react into the network, as discussed above in connection with the Saam patent, or are too volatile to be retained in the composition as it cures. Furthermore, long-chain alkyl alcohols which are also suggested as solvents would tend to undergo an exchange reaction with the hydrolyzable groups on silicon to effectively cap the polymer with a long-chain alkoxy group which is not readily hydrolyzed, thereby interfering with the intended moisture cure.
Although the above publications generally disclose moisture-curable systems based on organic polymers having hydrolyzable silyl groups and they recite various required or optional ingredients, one skilled in the art is not apprised of the specific requirements needed to prepare a stable, one-part composition which exhibits relatively rapid deep-section cure upon exposure to moisture, as discovered by the applicants.